Chris de Freitas: We need to be listening to science

We are often told we must cut carbon dioxide emissions drastically and without delay. The first urgent call came over 15 years ago at the Earth Summit in Rio de Janeiro.

But despite innumerable international meetings since then, each one producing promises and agreements, emissions from almost every country in the world have continued to rise.

Given the potentially serious risks posed by human-caused global warming, it is a curious fact that almost everyone promises to make drastic cuts in emissions that no one will live up to. There are signs that this is set to continue.

A growing number of people believe that stopping global warming has become their lowest priority, according to a Pew Research Centre survey in the United States earlier this year. The same conclusion can be drawn from a recent opinion poll by the AA in New Zealand of 1300 of its members.

One reason for this trend could be the difficulty many have in reconciling apparently conflicting evidence put before them. For instance, according to the United Nation's World Meteorological Organisation, the official climate record shows there has been no global warming for the past decade, despite steadily rising carbon dioxide concentrations in the atmosphere.

Some climate scientists claim this is a sign of a changed temperature trend, others argue it is snapshot in a highly variable climate system. Those in the former group point to new evidence that an extended period of cooling has begun.

The Sun was more active during much of the 20th century than it was for the previous 1000 years. Now, however, the trend appears to have reversed.

Solar activity is exceptionally low and there is no sign that this may change in the near future.

According to Dean Pesnell of Nasa's Goddard Space Flight Center: "Researchers are now seeing the dimmest Sun in their records.

"The change is small, just a fraction of a per cent, but significant. Questions about effects on climate are natural if the Sun continues to dim."

Predictions of future climate are usually based on global climate models. Up until now, these models have failed to consider variable energy from the Sun. The significance of this cannot be overstated as the Sun is the only source of energy to power Earth's climate, so all global climate change is directly or indirectly linked to it.

There is little doubt that average annual global temperature has been generally trending upwards in line with the expectations of many climate scientists. The cause, however, is debatable since the trend started before modern industrialisation began pumping millions of tonnes of carbon dioxide into Earth's atmosphere.

From 1940 to 1980 during the post World War II industrial boom when carbon dioxide increased rapidly, there were 40 years of global cooling.

On the other hand, there was a distinct global warm period in mediaeval times when carbon dioxide levels were much lower than they are now.

Carbon dioxide concentration in the atmosphere is currently higher than at any time in the past 600,000 years, yet global temperatures were much higher during all the major warm interglacial periods that occurred during this time, despite much lower levels of carbon dioxide.

From all this it is clear that warming and carbon dioxide are not well correlated.

The greenhouse effect of carbon dioxide in the atmosphere is almost saturated, so that the effect of carbon dioxide on global temperature is already close to its maximum. Adding more has an ever decreasing effect.

To illustrate the process, compare painting over a glass window with thin paint. The first coat of paint reduces some light shining through; the second coat cuts out a little more. Beyond this additional coats have an ever decreasing effect.

In the light of the latest evidence, a new question is being asked: What is the basis for the claim that carbon dioxide is a major driver of global climate?

If it turns out that there is no basis, or that the evidence for it is weak, a new and perhaps more important question arises: Are carbon dioxide emissions unwelcome?

It's a well-known fact that carbon dioxide is food for plants, and that at current concentrations they are carbon dioxide-starved. Increased carbon dioxide has a pronounced fertiliser effect on plant growth. Plants convert the carbon dioxide into food and fuel. It keeps our forests and pastures healthy.

No one knows for sure what the future holds, but there are some good clues as to what's going on. It hinges on growing evidence that natural influences on climate are in fact stronger than any man-made greenhouse effect.

It may be premature to discard our anxiety over the threat of possible human-caused global warming, but this anxiety should not be based on ignorance of what science can tell us.

* Chris de Freitas is a climate scientist at the University of Auckland.